Tricyclodecenyl esters, particularly the C2-C4 esters derived from dicyclopentadiene, are ubiquitous perfume ingredients found in detergents, shampoos, deodorants, hard-surface cleaners, and other applications. The most common tricyclodecenyl ester, which is made by reacting dicyclopentadiene with acetic acid, is tricyclodecenyl acetate or “TCDA.” The product, which has a sweet, anise-like aroma, is marketed by several companies, including, for example, International Flavors & Fragrances (Cyclacet®), Quest (Jasmacyclene®), Millennium Specialty Chemicals (Navace™), and Symrise (Herbaflorat®).
Tricyclodecenyl esters are normally produced by acid-catalyzed addition of the corresponding carboxylic acids (acetic, propionic, butyric, isobutyric) to dicyclopentadiene (DCPD). A large excess of the carboxylic acid is typically used. Solvents, such as aromatic hydrocarbons, are often included. Many catalysts have been proposed, including perchloric acid (U.S. Pat. No. 2,814,639), sulfuric acid (U.S. Pat. No. 2,395,452), boron trifluoride and its addition complexes (U.S. Pat. Nos. 4,855,488 and 4,358,617), sulfamic acid (Catal. Lett. 96 (2004) 71), p-toluenesulfonic acid (Zh. Org. Khim. 31 (1995) 528 and Neftek. 37 (1997) 76), and perfluorinated acidic ion-exchange resins (DE 3,105,399).
U.S. Pat. No. 2,395,452 teaches to prepare TCDA using a 550% molar excess of acetic acid and dilute H2SO4 as the catalyst. More recently, Leitmannova et al. (Perf. Flav. 29 (2004) 20) optimized the sulfuric acid-catalyzed process and concluded that use of a 400% excess of acetic acid at 100° C. provides a favorable balance of product yield and reaction time. Unfortunately, however, acetic acid is relatively expensive and neutralization of the excess acid during workup generates a large amount of waste.
U.S. Pat. No. 4,855,488 teaches to prepare tricyclodecenyl esters from 93%-pure DCPD, a boron trifluoride catalyst, and at least a 200% molar excess of carboxylic acid. Instead of neutralizing the acid, the patentees teach to recover it using a costly stripping step. However, even distillation fails to provide a commercially acceptable product; therefore, the distilled material is further purified to remove traces of BF3 catalyst and acetic acid by passing it through a column containing marble and/or activated carbon.
DE 3,619,797 teaches a process for obtaining TCDA using 1.1 to 5 molar excess of acetic acid and an ion-exchange resin. The water content is 0.5-15% based on the reactor charge, and acetic anhydride is added before distillative workup. In addition to the well-known drawbacks of ion-exchange resins (e.g., cost, loss of activity upon recycle), the need to charge acetic anhydride further complicates the method.
Commercial processes for making tricyclodecenyl esters typically require relatively pure dicyclopentadiene. The commercial grade DCPD material normally used is 93-94% DCPD, although a lower grade (83-88% pure) is available. High-purity DCPD (>98%) is also used. Unfortunately, esters made from the lower grade DCPD may not meet acceptable odor standards or isomer ratio requirements.
Conventional wisdom indicates that carboxylic acids (e.g., acetic acid) used to make tricyclodecenyl esters must also meet minimum purity standards. Crude acetic acid can be recovered from esterification processes used to make fragrance components, but this material is often contaminated with 20 wt. % or more of acetic anhydride, α-pinene, limonene, acetate esters, and other impurities. Because distillation of such streams is costly, they are often simply discarded.
Recently, as reported in copending application Ser. No. 11/489,100, filed Jul. 18, 2006, I described an improved way to make tricyclodecenyl esters that overcomes many of the drawbacks reported above. In that process, approximately equimolar amounts (0.8 to 1.3 molar ratio) of a C2-C4 carboxylic acid and a dicyclopentadiene react in the presence of triflic acid under conditions effective to produce the tricyclodecenyl ester. The process gives tricyclodecenyl esters in good yield from dicyclopentadienes while avoiding the need to use a large excess of the carboxylic acid. Moreover, fragrance-quality TCDA can be made from an inexpensive grade of DCPD and/or recycled acetic acid. Despite the advantages, there is room for improvement in how the esters are isolated. Although a large excess of carboxylic acid is not used, the typical workup still consists of adding a generous proportion of brine, water, and aqueous caustic to neutralize triflic acid and all of the excess, unreacted acetic acid. After separating phases, often with difficulty because of the tendency of the aqueous and organic layers to emulsify, the organic phase is distilled to isolate the tricyclodecenyl ester. Unfortunately, the basic workup does not permit recovery and reuse of any excess carboxylic acid. Moreover, the voluminous, salt-containing aqueous waste stream requires disposal and is preferably avoided. Sticky salts pose a reactor-cleanout challenge. Thus, issues created by the use of a basic workup are preferably avoided.
In sum, a simple, economical way to make tricyclodecenyl esters is needed. A valuable process would avoid the need to use a large excess of the carboxylic acid and would avoid the tradeoffs of using solvents, ion-exchange resins, or aqueous base workups. Ideally, the process would afford high yields of fragrance-quality tricyclodecenyl esters, even if the dicyclopentadiene and/or carboxylic acid sources are relatively impure.